1. Field of the Invention
This invention relates generally to seismic prospecting, and more particularly to a method of performing seismic surveys on land.
2. Description of Related Art
In seismic exploration, to obtain information relating to the sub strata located below the earth's surface, seismic signals in the form of pressure or shock waves are induced into the earth. These shock waves propagate through the substram beneath the earth's surface where they are reflected by the subterranean interfaces back to the earth's surface. The reflected waves are detected by a plurality of spaced apart receivers placed on the earth's surface which convert the reflected waves into signals. A geophone or a group of geophones is typically used as a receiver. The signals are recorded to provide maps of the substram. Seismic sources, such as seismic vibrators and/or explosive devices, are used to produce shock waves.
In recent years, three dimensional ("3D") seismic surveys have become very common for they provide more comprehensive information about the earth's subsurface compared to the conventional two dimensional ("2D") surveys. However, 3D surveys require the use of a complex surveying geometry and produce significantly more dam compared to the two dimensional surveys. Three dimensional surveys are typically performed using what is called a "swath method." In the swath method, a plurality of very long (3000-6000 meters) receiver lines, each containing a plurality of uniformly spaced apart receivers (receiver stations), are placed in parallel on the earth's surface (terrain) which is to be surveyed. Each receiver defines a single receiver point on the receiver line. The data gathering equipment limitations and other economic considerations frequently define the number of receiver lines and the number of receivers on each line that can be used to perform the survey. After placing the receiver lines, a seismic source such as a seismic vibrator or an explosive device is activated at predetermined spaced-apart locations (source stations) to impart desired shock waves in the earth. The source stations are placed along source lines which run midway between the receivers.
A typical prior art three-dimensional survey geometry is shown in FIG. 1. A plurality of receiver lines 20a, 20b . . . 20r, each containing a plurality of equally spaced apart receivers 22 are placed in parallel on the earth's surface. A source is activated at predetermined source stations, such as point 24, placed along source lines or shot lines 26a, 26b . . . 26n which run perpendicular to the receiver lines 20a-20n. The source lines typically lie at the middle of adjacent receiver. This provides a symmetrical crossed-array geometry wherein shot lines are orthogonal to the receiver lines 20. The resultant seismic traces (seismic data) are recorded corresponding to the common midpoints ("CMP") for the source points and the receivers. Once the source has been activated at each of the predetermined source points and the resultant data recorded, the swath is moved to an adjacent terrain and the above-noted process is repeated.
In the prior art seismic survey geometries, the receivers on all the receiver lines are equidistant and all lines are symmetrically placed, i.e., the source lines are placed midway between the receivers, as shown in FIG. 1. Such a geometry provides seismic data (traces) for common midpoints, which correspond to cells or bins having the dimensions of one-half (1/2) the spacing between adjacent receivers on the receiver lines and one-half (1/2) the spacing between adjacent source points along the source lines. For example, if the receivers and the source points are each fifty (50) meters apart, each bin will be twenty-five by twenty-five (25.times.25) meters. And each bin has seismic data corresponding to one common midpoint. The resultant bins obtained from the survey geometry of FIG. 1 are shown in FIG. 1A. Each solid lined square 28 indicates the bin size and the center 29 of each such cell represents the common mid point associated with that bin.
During operation, each source point produces seismic traces for a row of common midpoints. As the source is moved, the midpoints overlap. The data for each midpoint is collected for all shots and the data common to a midpoint is added or stacked to obtain better definition of data for each common mid point. The spacing between the receivers and the shot points, i.e., the bin size, defines the spatial resolution. The spatial resolution of such prior art methods may be improved by reducing the bin size by decreasing the receivers and/or shot point spacings, which increases the equipment cost, operational time to perform the survey and the data processing time. Also in the prior art methods, the bins are sufficiently large and may not be appropriately combined ("macrobinned"), for example, when data from certain CMP's is either not recorded due to physical configuration difficulties or equipment failure.
It is therefore highly desirable to have a method of geophysical prospecting which provides small data bins (higher spatial resolution) compared to the prior art bins ("standard" cells or bins) without substantially increasing the cost of the equipment or requiring additional operational time while allowing to preserve the benefits of the standard cells.
The quality of result obtained from conventional seismic data processing techniques, such as stacking, dip moveout correction and migration depends upon the spatial interval (bin size) which is limited by the data gathering methods. When a smaller bin size is desired, the prior art survey would need to be performed again using a narrower grid (smaller receiver and/or source spacing), which is sometimes not possible due to the nature of the terrain or is cost prohibitive. On the other hand, it is highly desirable to perform seismic surveys having wider spaced grids but which provide spatial resolution equivalent to a narrower grid. Furthermore, it is highly desirable to have a method for performing seismic surveys which for a given equipment provides flexibility with the bin size without compromising spatial resolution and without increasing operational cost.
The present invention provides a method for performing seismic surveys which addresses the above-noted problems. The method provides smaller or fractional bins compared to the bins obtained using conventional methods, provides flexibility of manipulating bin sizes and enables the use of differently spaced receiver and source lines for performing seismic surveys.